Electrode for plasma torch with novel assembly method and enhanced heat transfer

ABSTRACT

Embodiments of the present invention are related to an electrode for a plasma arc torch, the electrode comprising a generally tubular outer wall, an end wall, and a protrusion. The end wall is joined to a distal end of the outer wall and supports an emissive element in a generally central region. The protrusion extends from the generally central region of the end wall and is configured to connect with an electrode holder by a releasable connection, wherein the protrusion is configured such that at least one coolant passage forms between the protrusion and the electrode holder when the electrode is connected with the electrode holder. In some embodiments, the releasable connection comprises a threaded connection, wherein the protrusion is threaded to releasably connect to a threaded coolant tube of the electrode holder. In other embodiments, at least one coolant passage is defined by the threaded connection.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of pending U.S. patent applicationSer. No. 12/957,695, filed Dec. 1, 2010, the entirety of whichapplication is incorporated by reference herein.

FIELD

Embodiments of the present invention relate to electrode assemblies forplasma arc torches and, in particular, to electrodes and electrodeholders held to each other or to the plasma arc torch by way of athreaded connection. Further, some embodiments relate to electrodeassemblies having defined passages for providing coolant to theelectrode.

BACKGROUND

Plasma arc torches are commonly used for the working of metal includingcutting, welding, surface treatment, melting and annealing. Such torchesinclude an electrode that supports an arc that extends from theelectrode to a workpiece in a transferred-arc mode of operation. Tofacilitate operation, current is passed to the electrode to create thearc, which heats the electrode to high temperatures, causing erosion andreduction in electrode life. Thus, it is conventional to surround thearc with a vortex flow of plasma gas, and in some torch designs theplasma gas and arc are surrounded by a flow of secondary fluid such as agas or water.

SUMMARY OF THE INVENTION

In an effort to improve the electrode life and reduce manufacturingcosts, embodiments of the present invention provide an electrode for aplasma arc torch with novel assembly method and enhanced heat transferproperties.

One example embodiment is an electrode for a plasma arc torch, theelectrode comprising a generally tubular outer wall, an end wall, and aprotrusion. The end wall is joined to a distal end of the outer wall andsupports an emissive element in a generally central region of the endwall. The protrusion extends from the generally central region of theend wall and is configured to connect with an electrode holder by areleasable connection, wherein the protrusion is configured such that atleast one coolant passage forms between the protrusion and the electrodeholder when the electrode is connected with the electrode holder. Insome embodiments, the releasable connection may comprise a threadedconnection and the protrusion may be threaded to releasably connect theprotrusion to a threaded coolant tube of the electrode holder. In otherembodiments at least one coolant passage may be defined by the threadedconnection.

Another embodiment of the present invention is a plasma arc torchcomprising a torch body, a nozzle supported adjacent one end of thetorch body, an electrode, and an electrode holder. The electrode holderis supported by the torch body and is configured to provide coolantthrough an interior of the electrode holder. The electrode comprises anend wall that supports an emissive element and a protrusion extendingfrom a generally central region of the end wall. The protrusion connectsto the electrode holder by a releasable connection, wherein at least onecoolant passage is formed between the protrusion and the electrodeholder. The at least one coolant passage allows coolant to flowtherethrough and impinge on the end wall of the electrode. In someembodiments, the releasable connection comprises a threaded connectionand the protrusion may be threaded to releasably connect to a threadedcoolant tube of the electrode holder. In other embodiments, the at leastone coolant passage is defined by the threaded connection.

Other embodiments of the present invention include an electrode assemblyfor a plasma arc torch. The electrode assembly comprising an electrodeand an electrode holder. The electrode comprises a generally tubularouter wall, an end wall joined to a distal end of the outer wall andsupporting an emissive element in a generally central region of the endwall, and a protrusion extending from the generally central region ofthe end wall. The electrode holder connects to the electrode by areleasable connection and comprises an inner coolant tube for providingcoolant to the electrode and an outer coolant tube surrounding the innercoolant tube for removing coolant from the electrode via a space betweenthe inner and outer coolant tubes. The protrusion of the electrode isconfigured to connect with the inner coolant tube of the electrodeholder by a releasable connection and at least one coolant passage formsbetween the protrusion of the electrode and the inner coolant tube ofthe electrode holder when the electrode is connected with the electrodeholder. In some embodiments, the releasable connection comprises athreaded connection and the protrusion may be threaded to releasablyconnect the protrusion to a threaded coolant tube of the electrodeholder. In other embodiments, the at least one coolant passage isdefined by the threaded connection.

Another embodiment of the present invention is a method for cooling anelectrode in a plasma arc torch, comprising the steps of connecting anelectrode to an electrode holder by a releasable connectiontherebetween. The electrode has an end wall supporting an emissiveelement and a protrusion extending from a generally central region ofthe end wall, wherein the protrusion is configured to connect with theelectrode holder by the releasable connection. The method furthercomprises providing coolant through a coolant tube of the electrodeholder and through at least one coolant passage defined by thereleasable connection such that the end wall of the electrode isimpinged by the coolant.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a cross-sectioned side view of a conventional shielding gasplasma arc torch illustrating an electrode assembly as used in the priorart;

FIG. 2 is a cross-sectioned side view of the torch taken along adifferent section from FIG. 1 to illustrate coolant flow therethrough;

FIG. 3 is an enlarged cross-sectioned view of the lower portion of thetorch as seen in FIG. 1 and illustrating the conventional electrodeassembly;

FIG. 4 is an enlarged cross-sectioned view of the lower portion of aplasma arc torch illustrating one example embodiment of an electrodeassembly, in accordance with some embodiments discussed herein;

FIG. 5 is a cross-sectioned view of the electrode assembly of FIG. 4, inaccordance with some embodiments discussed herein;

FIG. 6 is an enlarged cross-sectioned view of the lower portion of theelectrode assembly of FIG. 4, in accordance with some embodimentsdiscussed herein;

FIG. 7 is an enlarged cross-sectioned view of an electrode of theelectrode assembly of FIG. 4, in accordance with some embodimentsdiscussed herein;

FIG. 7A is a perspective view of the electrode of FIG. 7, in accordancewith some embodiments discussed herein;

FIG. 8 is an enlarged cross-sectioned view of the lower portion of anelectrode holder of the electrode assembly of FIG. 4, in accordance withsome embodiments discussed herein;

FIG. 9 is an enlarged cross-sectioned view of the lower portion ofanother example embodiment of an electrode assembly, in accordance withsome embodiments discussed herein;

FIG. 10 is an enlarged cross-sectioned view of an electrode of theelectrode assembly of FIG. 9, in accordance with some embodimentsdiscussed herein;

FIG. 10A is a perspective view of the electrode of FIG. 10, inaccordance with some embodiments discussed herein;

FIG. 11 is an enlarged cross-sectioned view of the lower portion of anelectrode holder of the electrode assembly of FIG. 9, in accordance withsome embodiments discussed herein;

FIG. 12 is an enlarged cross-sectioned view of the lower portion ofanother embodiment of an electrode assembly, in accordance with someembodiments discussed herein;

FIG. 13 is an enlarged cross-sectioned view of an electrode of theelectrode assembly of FIG. 12, in accordance with some embodimentsdiscussed herein;

FIG. 13A is a perspective view of the electrode of FIG. 13, inaccordance with some embodiments discussed herein;

FIG. 14 is an enlarged cross-sectioned view of an electrode holder ofthe electrode assembly of FIG. 12, in accordance with some embodimentsdiscussed herein; and

FIG. 15 is a cross-sectioned view of another embodiment of an electrodeassembly, in accordance with some embodiments discussed herein.

DETAILED DESCRIPTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the inventions are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

Plasma arc torches often utilize electrodes that comprise an elongatetubular member composed of a material of high thermal conductivity(e.g., copper, copper alloy, silver, etc.) The forward or discharge endof the tubular electrode includes a bottom end wall having an emissiveelement embedded therein that supports the arc. The opposite end of theelectrode holds the electrode in the torch by way of a releasableconnection (e.g., threaded connection) to an electrode holder. Theelectrode holder is typically an elongate structure held to the torchbody by a threaded connection at an end opposite the end at which theelectrode is held. The electrode holder and the electrode define athreaded connection for holding the electrode to the electrode holder.

The emissive element of the electrode is composed of a material that hasa relatively low work function, which is defined in the art as thepotential step, measured in electron volts (eV), which promotesthermionic emission from the surface of a metal at a given temperature.In view of this low work function, the element is thus capable ofreadily emitting electrons when an electrical potential is appliedthereto. Commonly used emissive materials include hafnium, zirconium,tungsten, and alloys thereof.

A nozzle surrounds the discharge end of the electrode and provides apathway for directing the arc towards the workpiece. To ensure that thearc is emitted through the nozzle and not from the nozzle surface duringregular, transferred-arc operation, the electrode and the nozzle aremaintained at different electrical potential relative to each other.Thus, it is important that the nozzle and the electrode are electricallyseparated, and this is typically achieved by maintaining a predeterminedphysical gap between the components. The volume defining the gap is mosttypically filled with flowing air or some other gas used in the torchoperation.

The heat generated by the plasma arc is great. The torch component thatis subjected to the most intense heating is the electrode. To improvethe service life of a plasma arc torch, it is generally desirable tomaintain the various components of the torch at the lowest possibletemperature notwithstanding this heat generation. In some torches, apassageway or bore is formed through the electrode holder, and a coolantsuch as water is circulated through the passageway to internally coolthe electrode.

Even with the water-cooling, the electrode has a limited life span andis considered a consumable part. Thus, in the normal course ofoperation, a torch operator must periodically replace a consumedelectrode by first removing the nozzle and then unthreading theelectrode from the electrode holder. A new electrode is then screwedonto the electrode holder and the nozzle is reinstalled so that theplasma arc torch can resume operation.

Other considerations for electrode design include constraints on thethreaded connection between the electrode holder and the electrode. Forexample, the threaded connection must be structurally strong enough tosecurely hold the electrode to the electrode holder. Additionally, aconsiderable current is passed through the electrode holder to theelectrode, in some cases up to 1,000 amperes of cutting current. Thus,the threaded connection should provide sufficient contact surface areabetween the electrode and the electrode holder to allow this current topass through. Finally, the cost of manufacturing the electrode should beas small as possible, especially because the electrode is a frequentlyreplaced consumable part.

Thus, there is a need to increase the useful life of the electrode bymore efficient ways to provide coolant, while maintaining low cost ofmanufacture for the electrode and electrode holder.

The following discussion with respect to FIGS. 1-3 describes a priorplasma arc torch that would benefit from the invention. A plasma arctorch 300 using an electrode and electrode holder according to someembodiments of the present invention is illustrated in FIG. 4. Thus,embodiments of the present invention are described in greater detailwith respect to FIGS. 4-15.

FIGS. 1-3 show a prior plasma arc torch 10. The torch 10 is a shieldinggas torch, which provides a swirling curtain or jet of shielding gassurrounding the electric arc during a working mode of operation of thetorch. The torch 10 includes a generally cylindrical upper or rearinsulator body 12 which may be formed of a potting compound or the like,a generally cylindrical main torch body 14 connected to the rearinsulator body 12 and generally made of a conductive material such asmetal, a generally cylindrical lower or front insulator body 16connected to the main torch body 14, an electrode holder assembly 18extending through the main torch body 14 and front insulator body 16 andsupporting an electrode 20 at a free end of the electrode holderassembly, and a nozzle assembly 22 connected to the insulator body 16adjacent the electrode 20.

A plasma gas connector tube 24 extends through the rear insulator body12 and is connected by screw threads (not shown) into a plasma gaspassage 26 of the main torch body 14. The plasma gas passage 26 extendsthrough the main torch body 14 to a lower end face 28 thereof forsupplying a plasma gas (sometimes referred to as a cutting gas), such asoxygen, air, nitrogen, or argon, to a corresponding passage in theinsulator body 16.

A shielding gas connector tube 30 extends through the rear insulatorbody 12 and is connected by screw threads into a shielding gas passage32 of the main torch body 14. The shielding gas passage 32 extendsthrough the main torch body 14 to the lower end face 28 for supplying ashielding gas, such as argon or air, to a corresponding passage in theinsulator body 16.

The insulator body 16 has an upper end face 34 that abuts the lower endface 28 of the main torch body. A plasma gas passage 36 extends throughthe insulator body 16 from the upper end face 34 into a cylindricalcounterbore 38 in the lower end of the insulator body 16. As furtherdescribed below, the counterbore 38, together with the upper end of thenozzle assembly 22, forms a plasma gas chamber 40 from which plasma gasis supplied to a primary or plasma gas nozzle of the torch. As such,plasma gas from a suitable source enters the plasma gas chamber 40 byflowing through the plasma gas connector tube 24, through the plasma gaspassage 26 in the main torch body 14, into the plasma gas passage 36 ofthe insulator body 16, which is aligned with the passage 26, and intothe chamber 40.

The nozzle, which is illustrated as a two-part nozzle assembly 22,includes an upper nozzle member 42, which has a generally cylindricalupper portion slidingly received within a metal insert sleeve 44 that isinserted into the counterbore 38 of the insulator body 16. An O-ring 46seals the sliding interconnection between the upper nozzle member 42 andthe metal insert sleeve 44. A lower nozzle tip 48 of generallyfrustoconical form also forms a part of the nozzle assembly 22, and isthreaded into the upper nozzle member 42. The lower nozzle tip 48includes a nozzle exit orifice 50 at the tip end thereof. The lowernozzle tip 48 and upper nozzle member 42 could alternatively be formedas one unitary nozzle. In either configuration, the nozzle channels theplasma gas from a larger distal opening 49 to the exit orifice 50. Aplasma gas flow path thus exists from the plasma gas chamber 40 throughthe nozzle chamber 41 for directing a jet of plasma gas out the nozzleexit orifice 50 to aid in performing a work operation on a workpiece.

The plasma gas jet preferably has a swirl component created, in a knownmanner, by a hollow cylindrical ceramic gas baffle 52 partially disposedin a counterbore recess 54 of the insulator body 16. A lower end of thebaffle 52 abuts an annular flange face of the upper nozzle member 42.The baffle 52 has non-radial holes (not shown) for directing plasma gasfrom the plasma gas chamber 40 into a lower portion of the nozzlechamber 41 with a swirl component of velocity.

The electrode holder assembly 18 includes a tubular electrode holder 56which has its upper end connected by threads 11 within a blind axialbore 58 in the main torch body 14. The electrode holder 56 is somewhatconsumable, although usually less so than the electrode itself, and thusthe electrode holder and the axial bore 58 can also be provided with athreaded connection according to the present invention as discussedbelow. The upper end of electrode holder 56 extends through an axialbore 60 formed through the insulator body 16, and the lower end of theelectrode holder 56 includes an enlarged internally screw-threadedcoupler 62 which has an outer diameter slightly smaller than the innerdiameter of the ceramic gas baffle 52 which is sleeved over the outsideof the coupler 62. The electrode holder 56 also includes internal screwthreads spaced above the coupler 62 for threadingly receiving a coolanttube 64 which supplies coolant to the electrode 20, as further describedbelow, and which extends outward from the axial bore of the insulatorbody 16 into the central passage of the electrode 20. To preventimproper disassembly or reassembly of the coolant tube 64 and theelectrode holder 56, the screw thread connection between those items maybe cemented or otherwise secured together during manufacture to form aninseparable electrode holder assembly 18. The electrode 20 may be of thetype described in U.S. Pat. No. 5,097,111, assigned to the assignee ofthe present application, and incorporated herein by reference.

The prior art electrode 20 comprises a cup-shaped body whose open upperend is threaded by screw threads 63 into the coupler 62 at the lower endof the electrode holder 56, and whose capped lower end is closelyadjacent the lower end of the coolant tube 64. A coolant circulatingspace exists between the inner surface of the wall of the electrode 20and the outer surface of the wall of the coolant tube 64, and betweenthe outer surface of the wall of the coolant tube 64 and the innersurface of the wall of the electrode holder 56. The electrode holder 56includes a plurality of holes 66 for supplying coolant from the spacewithin the electrode holder to a space 68 between the electrode holderand the inner wall of the axial bore 60 in the insulator body 16. A seal69 located between the holes 66 and the coupler 62 seals against theinner wall of the bore 60 to prevent coolant in the space 68 fromflowing past the seal 69 toward the coupler 62. A raised annular rib ordam 71 on the outer surface of the electrode holder 56 is located on theother side of the holes 66 from the seal 69, for reasons which will bemade apparent below. A coolant supply passage 70 (FIG. 2) extendsthrough the insulator body from the space 68 through the outercylindrical surface of the insulator body 16 for supplying coolant tothe nozzle assembly 22, as further described below.

During starting of the torch 10, a difference in electrical voltagepotential is established between the electrode 20 and the nozzle tip 48so that an electric arc forms across the gap therebetween. Plasma gas isthen flowed through the nozzle assembly 22 and the electric arc is blownoutward from the nozzle exit orifice 50 until it attaches to aworkpiece, at which point the nozzle assembly 22 is disconnected fromthe electric source so that the arc exists between the electrode 20 andthe workpiece. The torch is then in a working mode of operation.

For controlling the work operation being performed, it is known to use acontrol fluid such as a shielding gas to surround the arc with aswirling curtain of gas. To this end, the insulator body 16 includes ashielding gas passage 72 that extends from the upper end face 34 axiallyinto the insulator body, and then angles outwardly and extends throughthe cylindrical outer surface of the insulator body. A nozzle retainingcup assembly 74 surrounds the insulator body 16 to create a generallyannular shielding gas chamber 76 between the insulator body 16 and thenozzle retaining cup assembly 74. Shielding gas is supplied through theshielding gas passage 72 of the insulator body 16 into the shielding gaschamber 76.

The nozzle retaining cup assembly 74 includes a nozzle retaining cupholder 78 and a nozzle retaining cup 80 which is secured within theholder 78 by a snap ring 81 or the like. The nozzle retaining cup holder78 is a generally cylindrical sleeve, preferably formed of metal, whichis threaded over the lower end of a torch outer housing 82 whichsurrounds the main torch body 14. Insulation 84 is interposed betweenthe outer housing 82 and the main torch body 14. The nozzle retainingcup 80 preferably is formed of plastic and has a generally cylindricalupper portion that is secured within the cup holder 78 by the snap ring81 and a generally frustoconical lower portion which extends toward theend of the torch and includes an inwardly directed flange 86. The flange86 confronts an outwardly directed flange 88 on the upper nozzle member42 and contacts an O-ring 90 disposed therebetween. Thus, in threadingthe nozzle retaining cup assembly 74 onto the outer housing 82, thenozzle retaining cup 80 draws the nozzle assembly 22 upward into themetal insert sleeve 44 in the insulator body 16. The nozzle assembly 22is thereby made to contact an electrical contact ring secured within thecounterbore 38 of the insulator body 16. More details of the electricalconnections within the torch can be found in commonly-owned U.S. Pat.No. 6,215,090, which is incorporated by reference herein in itsentirety.

The nozzle retaining cup 80 fits loosely within the cup holder 78, andincludes longitudinal grooves 92 in its outer surface for the passage ofshielding gas from the chamber 76 toward the end of the torch.Alternatively or additionally, grooves (not shown) may be formed in theinner surface of the cup holder 78. A shielding gas cup 94 of generallyfrustoconical form concentrically surrounds and is spaced outwardly ofthe lower nozzle tip 48 and is held by a shield retainer 96 that isthreaded over the lower end of the cup holder 78. A shielding gas flowpath 98 thus extends from the longitudinal grooves 92 in retaining cup80, between the shield retainer 96 and the retaining cup 80 and uppernozzle member 42, and between the shielding gas cup 94 and the lowernozzle tip 48.

The shielding gas cup 94 includes a diffuser 100 that in known mannerimparts a swirl to the shielding gas flowing into the flow path betweenthe shielding gas cup 94 and the lower nozzle tip 48. Thus, a swirlingcurtain of shielding gas is created surrounding the jet of plasma gasand the arc emanating from the nozzle exit orifice 50.

With primary reference to FIG. 2, the coolant circuits for cooling theelectrode 20 and nozzle assembly 22 are now described. The torch 10includes a coolant inlet connector tube 112 that extends through therear insulator body 12 and is secured within a coolant inlet passage 114in the main torch body 14. The coolant inlet passage 114 connects to thecenter axial bore 58 in the main torch body. Coolant is thus suppliedinto the bore 58 and thence into the internal passage through theelectrode holder 56, through the internal passage of the coolant tube64, and into the space between the tube 64 and the electrode 20. Heat istransferred to the liquid coolant (typically water or antifreeze) fromthe lower end of the electrode (from which the arc emanates) and theliquid then flows through a passage between the lower end of the coolanttube 64 and the electrode 20 and upwardly through the annular spacebetween the coolant tube 64 and the electrode 20, and then into theannular space between the coolant tube 64 and the electrode holder 18.

The coolant then flows out through the holes 66 into the space 68 andinto the passage 70 through the insulator body 16. The seal 69 preventsthe coolant in the space 68 from flowing toward the coupler 62 at thelower end of the holder 56, and the dam 71 substantially preventscoolant from flowing past the dam 71 in the other direction, althoughthere is not a positive seal between the dam 71 and the inner wall ofthe bore 60. Thus, the coolant in space 68 is largely constrained toflow into the passage 70. The insulator body 16 includes a groove orflattened portion 116 that permits coolant to flow from the passage 70between the insulator body 16 and the nozzle retaining cup 80 and into acoolant chamber 118 which surrounds the upper nozzle member 42. Thecoolant flows around the upper nozzle member 42 to cool the nozzleassembly.

Coolant is returned from the nozzle assembly via a second groove orflattened portion 120 angularly displaced from the portion 116, and intoa coolant return passage 122 in the insulator body 16. The coolantreturn passage 122 extends into a portion of the axial bore 60 that isseparated from the coolant supply passage 70 by the dam 71. The coolantthen flows between the electrode holder 56 and the inner wall of thebore 60 and the bore 58 in the main torch body 14 into an annular space126 which is connected with a coolant return passage 128 formed in themain torch body 14, and out the coolant return passage 128 via a coolantreturn connector tube 130 secured therein. Typically, returned coolantis recirculated in a closed loop back to the torch after being cooled.

In use, and with reference to FIG. 1, one side of an electricalpotential source 210, typically the cathode side, is connected to themain torch body 12 and thus is connected electrically with the electrode20, and the other side, typically the anode side, of the source 210 isconnected to the nozzle assembly 22 through a switch 212 and a resistor214. The anode side is also connected in parallel to the workpiece 216with no resistor interposed therebetween. A high voltage and highfrequency are imposed across the electrode and nozzle assembly, causingan electric arc to be established across a gap therebetween adjacent theplasma gas nozzle discharge. Plasma gas is flowed through the nozzleassembly to blow the pilot arc outward through the nozzle dischargeuntil the arc attaches to the workpiece. The switch 212 connecting thepotential source to the nozzle assembly is then opened, and the torch isin the transferred arc mode for performing a work operation on theworkpiece. The power supplied to the torch is increased in thetransferred arc mode to create a cutting arc, which is of a highercurrent than the pilot arc.

Example embodiments of the present invention are illustrated in FIGS.4-15. FIG. 4 shows one example embodiment of a plasma arc torch 300comprising an electrode holder assembly 318 and novel threadedconnection. The electrode holder assembly 318 comprises an electrodeholder 356 and an electrode 320. Although illustrated herein with atorch that uses a high-frequency pilot signal to start an arc, theelectrode and electrode holder according to the invention can also beused with blowback-type torches.

With reference to FIGS. 4-6, the electrode holder 356 is tubular andcomprises an upper end connected by threads 11 within the blind axialbore in the main torch body, as described above, and a lower endconnected to the electrode 320. The electrode holder 356 comprises aninner coolant tube 352 and an outer coolant tube 354. The inner coolanttube 352 supplies coolant to the electrode 320. The outer coolant tube354 is generally tubular shaped and annularly surrounds the innercoolant tube 352. The outer coolant tube 354 is configured to removecoolant from the electrode 320. The electrode holder 356 can be formedfrom a variety of different electrically conductive materials, but inone embodiment the electrode holder 356 is made of brass or a brassalloy.

The electrode 320, shown in cross-sectional view in FIG. 7 andperspective view in FIG. 7A, is generally cup-shaped. In the depictedembodiment, the electrode 320 comprises an outer wall 335, an end wall330, and a protrusion 325. The outer wall 335 is generally tubularshaped and, with reference to FIG. 6, may be configured to engage withthe electrode holder 356 such that an outer passage 337 is formedbetween the exterior surface of the inner coolant tube 352 and theinterior surface of the outer wall 335, allowing coolant to pass throughthe outer passage 337 to the outer coolant tube 354. The end wall 330joins to a distal end of the outer wall 335 and supports an emissiveelement 332 in a generally central region of the end wall 330. Theprotrusion 325 extends from the generally central region of the end wall330 and is configured to connect with the electrode holder 356 by areleasable connection, such as a threaded connection shown in thedepicted embodiment. The electrode 320 can be formed from a variety ofdifferent electrically conductive materials, but in one embodiment theelectrode 320 comprises a body made of copper or a copper alloy.

With reference to FIGS. 6 and 7, threads 310 secure the electrode 320 tothe electrode holder 356. In the depicted embodiment, the inner coolanttube 352 of the electrode holder 356 has a female threaded portion 317formed therein on the interior surface of the inner coolant tube 352.The protrusion 325 of the electrode 320 has a male threaded portion 319formed thereon on the exterior surface of the protrusion 325. The femalethreaded portion 317, shown in FIG. 8, may be formed on the lower end ofthe inner coolant tube 352 and configured to releasably receive the malethreaded portion 319 of the protrusion 325. The male threaded portion319 may comprise threads 310 configured as any type of thread, such as adouble start screw thread, a metric screw thread, a unified screwthread, a British standard pipe thread, a Whitworth screw thread, or ascrew thread having a stub acme profile as described in U.S. Pat. No.7,081,597, assigned to assignee of the present invention, and which ishereby incorporated herein by reference. The female threaded portion 317may also be configured with a thread profile to match a male threadedportion 319 with threads 310 configured as any type of thread, such asthose screw threads listed above.

In some embodiments, the protrusion 325 may be further configured suchthat at least one coolant passage 360 forms between the protrusion 325and the electrode holder 356 when the electrode 320 is connected to theelectrode holder 356. In the depicted embodiment, the male threadedportion 319 and the female threaded portion 317 are configured withextra space between the threads 310 so that coolant can flow between thethreads 310. In particular, as shown in FIG. 6, the extra space can format least one coolant passage 360 between the protrusion 325 and theinner coolant tube 352 of the electrode holder 356. In the depictedembodiment, the coolant passage 360 comprises a helically extendingspace between the thread profile of the male threaded portion 319 on theprotrusion 325 and the thread profile of the female threaded portion 317on the inner coolant tube 352. Thus, coolant enters the coolant passage360 from the inner coolant tube 352, spirals around the coolant passage360 within the threads, and exits the coolant passage 360 to impinge theend wall 330. The coolant then flows into the outer passage 337 and awayfrom the electrode 320 through the outer coolant tube 354. These novelcoolant passages 360, which follow the threaded connection of theelectrode assembly 318, allow the flowing coolant to contact innersurfaces of the electrode for a greater amount of time relative to anelectrode such as shown in FIGS. 1-3. Additionally, higher coolantvelocity may be achieved, which will improve convective heat transferfrom the electrode to the coolant. The net result should be enhancedcooling of the electrode 320, which in turn should prolong the life ofthe electrode 320.

In the depicted embodiment, the inner coolant tube 352 of the electrodeholder 356 comprises a bottom end 353 extending generally toward theelectrode 320. Different configurations of the electrode assembly 318may require the bottom end 353 of the inner coolant tube 352 to bepositioned properly with respect to the electrode 320 for the plasma arctorch to function properly. For example, in some embodiments, an opening363 may be configured to allow coolant to flow from the coolant passage360 to the outer passage 337 formed between the inner coolant tube 352and the outer wall 335 of the electrode 320, thereby allowing thecoolant to be ultimately removed from the electrode 320 through theouter coolant tube 354. In the depicted embodiment, the opening 363 isconfigured as additional space between the bottom end 353 of the innercoolant tube 352 and the end wall 330 of the electrode 320. In someembodiments, this may be accomplished by configuring the female threadedportion 317 to extend only partially up the lower end of the innercoolant tube 352 such that the protrusion 325 can be screwed into thefemale threaded portion 317 for only a certain distance, ensuring theopening 363 to form. Alternatively or additionally, a stopper (notshown) can be positioned within the female threaded portion 317 toprevent the male threaded portion 319 of the protrusion 325 from beingadvanced past a certain point (i.e., the position of the stopper),thereby ensuring the opening 363 forms.

Alternatively or additionally, in other embodiments, the opening 363 maycomprise a slot (not shown) in the inner coolant tube 352 that connectsthe coolant passage 360 to the outer passage 337. The slot may beconfigured adjacent to the bottom of the coolant passage 360 such thatcoolant flows through the entire coolant passage 360, out of the slot,into the outer passage 337, and up through the outer coolant tube 354.In some embodiments, the slot can allow the coolant to flow through theelectrode assembly 318 with greater velocity, improving the convectiveheat transfer and prolonging the life of the electrode 320.

FIGS. 9-11 show another embodiment of the present invention, wherein anelectrode assembly 418 utilizes a double start screw thread for thereleasable connection between the electrode 420 and the electrode holder456. However, the electrode assembly 418 may be used in a plasma arctorch in similar manner as the electrode assembly 318 described abovewith respect FIGS. 4-8, as well as with other embodiments of the presentinvention as described herein.

In the depicted embodiment of FIG. 9, the electrode assembly 418comprises an electrode 420 and an electrode holder 456. The electrode420 comprises a protrusion 425 with a male threaded portion 419 definedby threads 410 that form a double-start screw thread. The electrodeholder 456 comprises an inner coolant tube 452 with a female threadedportion 417 configured with a thread profile that matches the doublestart screw thread of the male threaded portion 419 to allow for areleasable connection. Furthermore, the thread profile of the male andfemale threaded portions 419, 417, similar to the male and femalethreaded portions 319, 317 of the electrode assembly 318, may also format least one coolant passage 460. In the depicted embodiment, theelectrode assembly 418 comprises two coolant passages 460 and 460′,formed separately due to the thread profile of the double start screwthread. As such, in some embodiments, coolant may flow through bothcoolant passages 460, 460′ out of respective openings 463, 463′ and awayfrom the electrode 420 through the outer coolant passage 454. In thedepicted embodiment, the openings 463, 463′ comprise slots formed in theinner coolant tube 452.

FIGS. 12-14 show another embodiment of the present invention, whereinthe electrode assembly 518 comprises an electrode 520 and an electrodeholder 556 having a threaded connection between an exterior surface ofan inner coolant tube 552 and an interior surface of a protrusion 525.The electrode assembly 518 may also be used in a plasma arc torch insimilar manner as the electrode assembly 318 described with respect toFIGS. 4-8, as well as with other embodiments of the present invention asdescribed herein.

With reference to FIG. 12, the electrode 520 comprises an annularlyshaped protrusion 525 having a female threaded portion 517. Theelectrode holder 556 comprises an inner coolant tube 552 having a malethreaded portion 519 comprising threads 510. The thread profile of themale threaded portion 519 and the corresponding female threaded portion517 may be any type of screw thread, such as a double start screwthread, a metric screw thread, a unified screw thread, a Britishstandard pipe thread, a Whitworth screw thread, or a screw thread havinga stub acme profile.

In the depicted embodiment, the inner coolant tube 552 and the centralbore of the protrusion 525 define an inner passage 536 configured toallow coolant to flow to the electrode 520. In some embodiments, theinner passage 536 defines a reservoir positioned directly above an endwall 530 and an emissive element 532 contained within the end wall 530.Thus, coolant in the reservoir will directly contact the portion of theelectrode 520 with the highest temperature (i.e., the portion near theemissive element 532).

Additionally, in the depicted embodiment, the electrode 520 furthercomprises at least one passage or slot 576. One end of the slot 576connects the inner passage 536 to an outer passage 537 defined betweenthe protrusion 525 and the outer wall 535. Therefore, the slot 576allows coolant to flow from the inner passage 536 to the outer passage537, so that the coolant can ultimately flow away from the electrodethrough the outer coolant tube 554 of the electrode holder 556.

FIG. 15 shows another embodiment of an electrode assembly 618 with anelectrode holder 656 and an attached electrode 620. In the depictedembodiment, the electrode 620 comprises a tubular outer wall 635 thatextends past the protrusion 625. Coolant can flow through the innercoolant tube 652 of the electrode holder 656 and around the protrusion625 to cool the electrode 620, as described in various embodimentsabove. Furthermore, the coolant can flow between the exterior surface ofthe inner coolant tube 652 and the interior surface of the outer wall635 to the outer coolant tube 654 for removal from the electrode 620.

Another embodiment of the present invention includes a method forcooling an electrode in a plasma arc torch comprising providing coolantto the electrode through various embodiments of the invention asdescribed herein. In particular, the method may comprise the steps ofconnecting an electrode to an electrode holder by a releasableconnection therebetween, the electrode having an end wall supporting anemissive element and a protrusion extending from a generally centralregion of the end wall, the protrusion being configured to connect withthe electrode holder by the releasable connection. The method mayfurther comprise providing coolant through a coolant tube of theelectrode holder and through at least one coolant passage defined by thereleasable connection such that the end wall of the electrode isimpinged by the coolant. In some embodiments, the method may furthercomprise removing coolant from the at least one coolant passage throughat least one slot adjacent to the coolant passage. The method may alsofurther comprise removing coolant from the electrode through an outercoolant tube defined in the electrode holder. In other embodiments, thestep of providing coolant through the at least one coolant passagecomprises passing coolant through a helically extending space between athread profile on the protrusion of the electrode body and a threadprofile on the coolant tube of the electrode holder.

Embodiments of the present invention as described herein addressreleasable connections and the issue of heat transfer for electrodecooling methods. In particular, some embodiments utilize coolingpassages formed between the thread profile of the electrode and theinner coolant tube of the electrode holder to increase coolant flowvelocity and increase the surface area used for heat transfer betweenthe electrode and the coolant. Other embodiments described herein alsoadvantageously utilize slots to facilitate coolant flow. In fact, thecombination of the coolant passages and the slots has shown to increaseflow velocity by three fold. Thus, embodiments of the present inventionimprove heat transfer by increasing flow velocity and increasing thesurface area of the electrode that interacts with the coolant, therebyincreasing the useable life of the electrode in a plasma arc torch.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included herein. Although specific terms are employedherein, they are used in a generic and descriptive sense only and notfor purposes of limitation.

What is claimed is:
 1. An electrode for a plasma arc torch, theelectrode comprising: a generally tubular outer wall; an end wall joinedto a distal end of the outer wall and supporting an emissive element ina generally central region of the end wall; a protrusion extending fromthe generally central region of the end wall, the protrusion centrallydisposed within the generally tubular outer wall, the protrusioncomprising threads for securing the electrode to threads of theelectrode holder; and a coolant passage formed between the protrusionand the electrode holder when the electrode is connected with theelectrode holder.
 2. The electrode of claim 1, wherein the protrusionthreads comprise female threads for securing the electrode to malethreads of the electrode holder.
 3. The electrode of claim 1, whereinthe coolant passage comprises a slot.
 4. The electrode of claim 3,wherein the slot comprises a radially disposed opening coupling an innerpassage of the protrusion to an outer passage of the protrusion.
 5. Theelectrode of claim 4, wherein the slot comprises a plurality of radiallydisposed openings coupling the inner passage of the protrusion to anouter passage of the protrusion, the plurality of radially disposedopenings positioned in spaced apart relation about a circumference ofthe protrusion.
 6. The electrode of claim 1, wherein at least a portionof the coolant passage is defined by the threaded connection between theelectrode and the electrode holder.
 7. The electrode of claim 1, whereinthe threaded connection is configured as one of a double start screwthread, a metric screw thread, a unified screw thread, a Britishstandard pipe thread, a Whitworth screw thread, and a screw threadhaving a stub acme profile.
 8. The electrode of claim 1, wherein theprotrusion is further configured such that a gap forms between the endwall and the coolant tube of the electrode holder when the electrode isconnected to the electrode holder.
 9. The electrode of claim 1, whereinthe protrusion is annular and comprises an interior surface, and whereinat least a portion of the interior surface of the protrusion isconfigured to connect with the coolant tube of the electrode holder bythe threaded connection.
 10. An electrode assembly for a plasma arctorch, comprising: an electrode comprising; a generally tubular outerwall; an end wall joined to a distal end of the outer wall andsupporting an emissive element in a generally central region of the endwall; and a protrusion extending from the generally central region ofthe end wall, at least a portion of the protrusion being centrallydisposed within the generally tubular outer wall of the electrode; andan electrode holder connected to the electrode, the electrode holdercomprising: an inner coolant tube for providing coolant to theelectrode; and an outer coolant tube surrounding the inner coolant tubefor removing coolant from the electrode via a space between the innerand outer coolant tubes; wherein the protrusion of the electrodeincludes threads for connecting with threads of the inner coolant tubeof the electrode holder, wherein a coolant passage is formed between theprotrusion and the electrode holder when the electrode is connected withthe electrode holder.
 11. The electrode assembly of claim 10, whereinthe protrusion threads comprise female threads for securing theelectrode to male threads of the electrode holder.
 12. The electrodeassembly of claim 10, wherein the coolant passage comprises a slot. 13.The electrode assembly of claim 12, wherein the slot comprises aradially disposed opening coupling an inner passage of the protrusion toan outer passage of the protrusion.
 14. The electrode assembly of claim13, wherein the slot comprises a plurality of radially disposed openingscoupling the inner passage of the protrusion to an outer passage of theprotrusion, the plurality of radially disposed openings positioned inspaced apart relation about a circumference of the protrusion.
 15. Theelectrode assembly of claim 10, wherein the protrusion is annular andcomprises an interior surface, and wherein at least a portion of theinterior surface of the protrusion is configured to connect with thecoolant tube of the electrode holder by the threaded connection.
 16. Theelectrode assembly of claim 10, wherein the protrusion is furtherconfigured such that a gap forms between the end wall and the coolanttube of the electrode holder when the electrode is connected to theelectrode holder.
 17. A method for cooling an electrode in a plasma arctorch, comprising the steps of: connecting an electrode to an electrodeholder by a threaded connection therebetween, the electrode having anend wall supporting an emissive element and a protrusion extending froma generally central region of the end wall, the protrusion centrallydisposed within a generally tubular outer wall of the electrode, theprotrusion including threads for securing the electrode to threads ofthe electrode holder; and providing coolant through a coolant tube ofthe electrode holder and through at least one coolant passage definedbetween the protrusion and the electrode holder such that the end wallof the electrode is impinged by the coolant.
 18. The method of claim 17,wherein the coolant passage comprises a slot, the method comprisingmoving coolant through the slot.
 19. The method of claim 17, furthercomprising removing coolant from the electrode through an outer coolanttube defined in the electrode holder.
 20. The method of claim 17,wherein the coolant passage comprises a plurality of radially disposedslots, the method comprising moving coolant through the plurality ofslots such that the end wall of the electrode is impinged by thecoolant.